Nonlinear resistance bridge utilizing transformer providing time delay compensation



March 4, 1958 2,825,864 NONLINEAR RESISTANCE BRIDGE UTILIZING TRANSFORMEFiled July 29, 1954 W F EAGAN R PROVIDING TIME DELAY COMPENSATION 2Sheets-Sheet 1 C 67 g! A c 11121:

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March 4, 1958 w. F. EAGAN 2,825,864

NONLINEAR RESISTANCE BRIDGE UTILIZING TRANSFORMER PROVIDING TIME DELAYCOMPENSATION 2 Sheets-$heet 2 Filed July 29, 1954 NONLlNEAR RESHSTANQEBRIDGE UTILIZING TRANSFORMER PRGVIDl-[NG TIME DELAY COMPENSATION WilliamF. Eagan, West Allis, Wis,

assignor to Allis- Chalmers Manufacturing Company,

This invention relates in general to voltage error detecto'rs and inparticular to time delay compensating means for detectors utilizinglinear and nonlinear resistors.

Voltage error detector bridge circuits utilizing linear and nonlinearresistors are known in which the inherent time delay of the nonlinearresistors is compensated for by capacitors connected across some of thebranches of the bridge circuit.

A disadvantage of these known circuits is that they cannot readily beused in aircraft at high altitudes where low temperatures exist. This isbecause electrolytic capacitors are not reliable at low temperatures andoil filled capacitors are too heavy and occupy too much volume. Neithertype of capacitor is therefore suited for use in a bridge circuitinstalled in an aircraft.

The invention overcomes this disadvantage by providing, in alinear-nonlinear resistor bridge circuit, a transformer having onewinding connected across the input junctions of the bridge and anotherwinding connected in series with the output junctions of the bridge. Thetransformer affords reliable time delay compensation at extremely lowtemperatures and is relatively small and light in weight.

It is therefore an object of this invention to provide a voltage errordetector of the linear-nonlinear resistor type having time delaycompensating means reliable at low temperatures and relatively light inweight and small in volume.

Other objects and advantages will appear from the following detaileddescription when read in connection with the accompanying drawings, inwhich:

Fig. 1 diagrammatically illustrates one embodiment of the invention asapplied to a rectifying system;

Fig. 2 shows in more detail, a saturable reactor shown in Fig. 1;

Fig. 3 is a graph illustrating the transfer characteristic'of thereactor shown in Fig. 2;

Fig. 4 shows, in more detail, a saturable reactor shown in Fig. 1;

Fig. 5 is a graph illustrating the transfer characteristic of thereactor shown in Fig. 4;

Fig. 6 shows, in more detail, a saturable reactor shown in Fig. 1;

Fig. 7 a is graph illustrating the transfer characteristic of thereactor shown in Fig. 6; and

Fig. 8 is a graph illustrating the transfer characteristic of a bridgecircuit shown in Fig. 1.

In the drawing, where an underlined reference numeral appears inproximity with a plurality of lower case reference letters, the numeralindicates a means comprising a plurality of elements and the elementsare indicated by the lower case reference letters. In the specification,these elements are identified by the reference numeral accompanied bythe reference letter.

Referring to Fig. l, the invention is shown embodied in a rectifyingsystem wherein a main rectifier 20 sup- Patent 0 plies direct current toa load device 11. The direct current terminals of the rectifier 20 areconnected to output terminals 12, 13 through a well known filteringarrangement comprising capacitor 14 and choke coils i6, 17 to impress adirect current voltage across output terminals 12, 13 for providing adirect current voltage source for load device 11 which is connectedacross terminals 12, 13 by means of cables or conductors i3, 19. Thealternating current terminals of the rectifier 20 are connected througha transformer 22 and the reactanc'e' windings of a s'a'turable reactor21 to an alternating current source of substantially constant voltagesuch as an alternating current" generator 23.

Saturable reactor 21 has a core 21a, a pair of reactance windings 21b, apair of rectifiers 210, a bias winting 21d and a signal winding 21c.Rectifiers 21c provide self-saturation or self-excitation for thereactor The bias winding 21d and signal winding 1212 act in oppositionto each other to determine the reactance of reactance windings 21b tocontrol the voltage applied to transformer 22 and rectifier 20 and thusto control the direct current voltage of rectifier 20 impressed acrossoutput terminals 12, 13. Bias winding 21d of reactor 21 is connected tothe direct current terminals of rectifying device 24 through anadjustable resistor 26. The alternating current terminals of rectifyingdevice 24' are connected to a transformer 27 which is energized byalte'rnating current generator 23. The bias winding 21d thus hasimpressed thereon a substantially constant unidirectional bias voltage,the value of which is determined by the setting of resistor 26. Signalwinding He is connected through an adjustable resistor 28 to the di rectcurrent terminals of a second rectifier 30. The direct current voltageoutput of rectifier 30 is controlled by a saturable reactor 31.

Reactor 31 has a core 31a, a pair of reactance windings 31b, a pair ofrectifiers 310, a current limit control winding 31, a voltage controlwinding 31c and a feedback winding 31). Reactance windings 31b andrectifiers 310 are serially connected with the alternating currentterminals of rectifier 30 to transformer 27. Signal winding 21s ofreactor 21 thus has impressed-thereon a unidirectional signal voltage,the magnitude of which depends upon the net effect of windings 31d, 31eof reactor 31. Reactors 21 and 31 are included in the voltage regulatorportion of the system. Winding 31s is the voltage responsive element andwinding 31d is the current limit responsive element of the voltageregulator.

The direct current output voltage of rectifier 30 is fed back intofeedback winding 31f through a capacitor 32 and an adjustable resistor33 to prevent the system from overshooting and oscillating during thevoltage recovery. The setting of resistor 33 determines the speed andmanner of recovery.

Voltage control winding 31c is connected to respond to the output of avoltage error detector comprising a bridge circuit 34 and a transformer36. The bridge circuit 34 comprises four resistors, at least one ofwhich is a nonlinear voltage dependent resistor. As shown, the bridgecircuit 34 has a pair of nonlinear voltagedependent resistors such astungsten filament light bulbs 34a connected in series with a pair oflinear or constant adjustable resistors 34b to form a bridge circuit 34.A first pair of opposite junctions 340 of the bridge circuit areconnected across output terminals 12, 13 of the rectifying system tohave the direct current voltage of rectifier 20 impressed on junctions34c. The constant-r sistors 34b may be adjusted to cause the voltageacross the other pair of opposite junctions 34a. to be approximatelyzero for the desired value of the controlled volt;-

age across terminals 12, 13 and to vary in magnitude and direction withvariations in the controlled voltage from the desired value.

A rheostat 35 may be used to caused the bridge circuit 34 to balance atany desired value of the controlled voltage which is impressed acrossjunctions 340. A reduction of the resistance of rheostat 35 causes thebridge circuit to balance at a lower value of the controlled voltage.

Transformer 36 has a first winding 36a connected in series with anadjustable resistor 37 across input junctions 34c, and a second winding36b connected in series with output junctions 34a, an adjustableresistor 38 and voltage control winding 31:; of saturable reactor 31.Voltage control winding 31e thus has impressed thereon a direct currentreversible polarity control voltage which varies in direction andmagnitude with variations in the controlled direct current voltage ofrectifier 20 appearing across output terminals 12, 13.

The time delay inherent in the nonlinear voltage dependent resistors 34ais compensated for by transformer 36. Any change in the input to thebridge circuit at junctions 34c is sensed by transformer winding 36a andis immediately reflected into transformer Winding 36b which is in serieswith junctions 34d and voltage control winding 31c. Voltage controlwinding 31:: thus senses the change without time delay. Resistor 37 isadjusted to obtain the desired speed of response. The disadvantage ofthe inherent time delay in the nonlinear resistors is thus overcome.

The voltage impressed on current limit control Winding 31d of reactor 31is controlled by a current limit device comprising a saturable reactor41 and a bridge circuit 47. Winding 31d is connected through anadjustable resistor 39 to the direct current terminals of a rectifier40. The direct current output voltage of rectifier 40 is controlled bysaturable reactor 41.

Reactor 41 has a core 41a, a pair of reactance windings 41b, a pair ofrectifiers 410, a saturating winding 41d and a damping winding 41c.Reactance windings 41b and rectifiers 410 are serially connected withthe alternating current terminals of rectifier 40 to transformer 27.There is thus impressed on current limit control winding 31d of reactor31 a unidirectional current limit control voltage, the magnitude ofwhich depends upon the net efiect of saturating winding 41d and dampingwinding 412.

Damping winding 41e is connected through an adjustable resistor 42 tothe output winding 43a of a damping transformer 43. An input winding 43cof this damping transformer is connected through an adjustable resistor44 to the direct current output terminals of rectifier 30. The conductorcarrying the direct current output of main rectifier 20 acts as anotherinput winding 43b for damping transformer 43. Transformer 43 thus senseschanges in the direct current in the load device 11 and changes in theoutput of rectifier 30 to impress a damping signal across dampingwinding 412. A rectifier 46 is connected across winding 41e to cause itto receive damping signals of one direction only.

Saturating winding 41d is connected to respond to the output of a bridgecircuit 47. The bridge circuit 47 comprises four resistors, at least oneof which is a nonlinear resistor. As shown, the bridge circuit 47 has apair of nonlinear resistors such as tungsten filament light bulbs 47aand a pair of linear or constant adjustable resistors 47b connectedtherewith in series to form a bridge circuit having a pair of oppositeinput junctions 47c and a pair of opposite output junctions 47d.

Rectifier 48 has its direct current terminals connected through resistor38 to input junctions 47c. The alternating circuit terminals ofrectifier 48 are fed from a current transformer 50 in the alternatingcurrent line that feeds main rectifier 20. Input junctions 47c thus haveimpressed thereacross from rectifier 48, a unidirectional voltagedependent upon the direct current in load device 11.

Input junctions 570 also are connected to the direct current terminalsof rectifier 3-9. The alternating current terminals of rectifier 49 areconnected to transformer 27. Input junctions 47c therefore also haveimpressed thereacross, from rectifier a substantially constantunidirectional reference voltage.

The output junctions .74! of the bridge circuit are connected tosaturating winding 41d of reactor all to impress thereon a reversibleunidirectional voltage dependent upon the value of the direct current inload device 11.

Fig. 2 shows in detail the connections for the main saturable reactor21. Bias winding 21d is wound and energized so that its ampere turnsoppose the ampere turns due to the direct current component in reactancewindings 21b and rectifiers 21c. Signal Winding 212 is Wound andenergized so that its ampere turns aid the ampere turns due to thedirect current component of the current in reactance windings 21b andrectifiers Zlc. Increases in current through signal windings 21e causeincreases in the direct current voltage of the main rectifier 2t) anddecreases of said current cause decreases of said voltage.

Fig. 3 is a graph showing the transfer characteristic 51 of saturablereactor 21. The abscissa represents the net ampere turns of windings21d, 212 and the ordinate represents the direct current voltage of mainrectifier 20.

Fig. 4 shows in detail the connections for saturable reactor 31. Currentlimit control winding 31d is wound and energized so that its ampereturns oppose the ampere turns due to the direct current component of thecurrent in reactance windings 311: due to rectifiers 31c. Voltagecontrol winding 31a may either aid or oppose the saturation of thereactor in response to the reversible unidirectional output voltage ofbridge circuit 34 which is impressed on Winding 312. When the directcurrent voltage of main rectifier 24B is below the desired value, theampere turns of winding 31:: act to increase the voltage impressed onsignal winding Zle of reactor 21. When the direct current voltage ofmain rectifier 20 is above the desired value, the ampere turns ofwinding 31a act to decrease the voltage impressed on signal winding 21cof reactor 21.

Fig. 5 is a graph showing the transfer characteristic curve 52 ofsaturable reactor 31. The abscissa represents the net ampere turns ofwindings 31d, 31e and the ordinate represents the signal voltageimpressed on winding 21a of reactor 21.

Fig. 6 shows the details of the connections for saturable reactor 41.Saturating winding 41d has impressed thereon a reversible unidirectionalvoltage and therefore its ampere turns may either aid or oppose theampere turns due to the direct current component in reactance windings41b due to rectifiers 41c and therefore may either increase or decreasethe unidirectional voltage impressed on current limit control winding31d of reactor 31. Damping winding 41c, because of rectifier 46connected thereacross has an appreciable effect only upon a decrease inthe current load device 11, whereupon its ampere turns act to decreasethe voltage of current limit control winding 31d of reactor 31.

Fig. 7 is a graph showing the transfer characteristic curve 53 ofsaturable reactor 41. The abscissa represents the ampere turns ofsaturating Winding 41d and the ordinate represents the voltage impressedon current limit control winding 31d of reactor 31.

Fig. 8 is a graph showing the transfer characteristic curve 54 of thecurrent limit bridge circuit 47. The abscissa represents the voltageacross input junctions 470 which is dependent upon the load current andthe ordinate represents the voltage across output junctions 4711 whichis impressed on saturating winding 41d of reactor 41. The referencevoltage impressed on input juncass-ease tions 47c by rectifier 49 isrepresented by 56. This reference voltage causes a voltage outputrepresented by 57 and prevents operation on the dash line portion of thecurve. As long as the voltage of rectifier 48 is equal to or less thanthat of rectifier 49, the bridge circuit operates at a point 76 on thecurve. When the voltage of rectifier 48 exceeds the reference voltage ofrectifier 49, the bridge circuit may operate at any point on the curveto the right of point 7d and the bridge output at output junctions 47dvaries in dependence upon the voltage of rectifier 43. For a voltage ofrectifier as represented by 58, which is the voltage corresponding tothe limit value of the load current, the bridge operates at point 80 tohave an output voltage represented by 59. For voltages of rectifier 48higher than voltage 58, the bridge output decreases to become zero atpoint 9%; and then reverses in direction and increases in magnitude forstill further increases in the voltage of rectifier 43. Points 71, 81and 91 on the curve 53 in Fig. 7 corresponds to points 70, Si) and 90 oncurve 54 in Fig. 8.

in the operation of the system, the direct current voltage of mainrectifier impressed across terminals 12, 1.3, is controlled according toso-called knee curve regulation to maintain the voltage across loaddevice 11 substantially constant as load current therethrough variesfrom zero up to a predetermined limit value.

The load device could be, for example, an airplane starting motor havinga low thermal capacity. The load current must therefore be carefullycontrolled so as not to overheat the motor during the starting period.When such a motor is thrown on the system, its current would be limitedonly by the total resistance of its armature circuit unless its terminalvoltage is reduced. As

the motor accelerates and develops a counter electromotive force, thevoltage can be increased and the current still held under thepredetermined limit value. The voltage will keep increasing until theknee of the voltage current curve is reached, at which point the loadcurrent drops off and the motor runs on almost constant voltage.

The direct current output voltage of the main rectifier 20 is controlledin the following manner.

Assume that the voltage across terminals 12, 13 falls below itspredetermined desired value. The bridge circuit 34 unbalances and anoutput voltage appears across output junctions 34d causing a controlvoltage to be impressed on voltage control winding 31s of reactor 31.This control voltage is in the direction to increase the voltageimpressed on signal winding lie of reactor 21 to thus increase thedirect current voltage of the main rectifier 20 until the predetermineddesired value is reached to thus bring the bridge circuit 34 back towardbalance. When the direct current voltage of main rectifier Zfi risesabove its predetermined desired value, the operation is the reverse ofthat just described.

Line drop compensation for the voltage drop in lines 13, 19 must beprovided to maintain the voltage across load device 11 constant in theface of varying current in the load device. Line drop compensation isprovided by the current limit device comprising bridge circuit 47 andsaturable reactor 41 when the load current is above a predeterminednormal value such as the value corresponding to point '70 in Fig. 8 andbelow a predermined limit value such as the value corresponding to point80 in Fig. 8. The transfer characteristic curve 53 of reactor 41, shownin Fig. 7, is such that between points 71 and 81, as the load current inload device 11 increases, the current limit control voltage impressed oncurrent limit control winding 31d decreases, thus allowing the signalvoltage impressed on signal winding 21a to increase to thus increase thedirect current voltage of the main rectifier 29. Line drop compensationis thus accomplished to enable the voltage regulator to maintain thevoltage of the load device constant in the face of increasing loadcurrent between a predetermined normal value and a predetermined, limitvalue.

Below the normal value of current, the current limit device maintains aconstant output to prevent the occurrence of the reverse of line dropcompensation which would occur if the bridge circuit 47 did not have arefence voltage impressed thereon by rectifier 49. Without the referencevoltage, the signal voltage impressed on signal winding 21c wouldincrease in response to increases of the load current between zero andthe predetermined normal value to accomplish the undesirable reverse ofline drop compensation and would thereby hinder the voltage regulatortending to prevent it from maintaining the load voltage constant.

When the load current reaches its limit value, rectifier 48 produces avoltage indicated at 58 which causes bridge circuit 47 to operate atpoint and saturable reactor 41 to operate at point 81. If the loadcurrent exceeds this limit value the output of reactor 41 increasesgreatly, causing the voltage impressed on current-limit control winding31d to increase greatly. This greatly decreases the voltage of signalwinding 21a to greatly decrease the direct current voltage of the mainrectifier 20 thereby limiting the load current to the predetermineddesired value.

The current limit device thus provides both current limit action andline drop compensation. The line drop compensation is efiective for loadcurrents in a range between a predetermined normal value and apredetermined limit value. This is the range where line dropcompensation is most needed because the current values are high. Theline drop compensation action stops and the current limiting actionbegins when the current exceeds the limit value.

Additional line drop compensation may be obtained by modulating thevoltage output across junctions 34d by the voltage drop in resistor 38.Resistor 38 is connected through rectifier 48 to current transformer 50and is thus traversed with a current proportional to the load current inload device 11. This modulation compensates for the voltage drop incables 18, 19 to enable the voltage regulator to maintain the voltage ofthe load substantially constant over the entire range of varying loadcurrent regardless of the voltage drop in the cables.

Features disclosed but not claimed herein are claimed in application ofWilliam F. Eagan and Roger L. Robertson, Serial No. 446,593, filed July29, 1954, now issued Patent U. S. 2,723,372. I

Although only one embodiment of the invention has been illustrated anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications -50 may be made therein without departing fromthe spirit of the invention and the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. A voltage error detector comprising four resistors connected inseries to form a bridge circuit having two pairs of opposite junctions,one of said resistors being a nonlinear voltage dependent resistor,means connecting one pair of said opposite junctions to a variablevoltage source, means connecting the other pair of said oppositejunctions to a voltage responsive element for impressing on said elementa voltage varying in magnitude with variations of the voltage of saidsource from a predetermined value, and time delay compensation meanscomprising a transformer having a first winding connected across saidone pair of opposite junctions and a second winding connected in serieswith said element.

2. A voltage error detector comprising four resistors connected inseries to form a bridge circuit having two pairs of opposite junctions,one of said resistors being a nonlinear voltage dependent resistor,means connecting one pair of said opposite junctions to a variableunidirectional voltage source, means connecting the other pair of saidopposite junctions to a voltage responsive element for impressing onsaid element a reversible unidirectional voltage varying in directionand magnitude with variations of the voltage of said source from apredetermined value, and time delay compensation means comprising atransformer having a first winding connected across said one pairoftopposite junctions and a second winding connected in series with saidelement.

3-. In combination, a first rectifier, a source of alternating currentfor energizing said first rectifier, a first saturable reactor having afirst reactance winding in series with said source and having a biaswinding and a signal winding acting in opposition for controlling thedirect current voltage of said first rectifier, means for impressing asubstantially constant unidirectional voltage on said bias winding, asecond saturable reactor having a second reactance winding, meansincluding a second rectifier connecting said source to said secondreactance winding in series with said signal winding for impressing aunidirectional signal voltage on said signal winding, said secondsaturable reactor having a voltage control winding for controlling themagnitude of said signal voltage, and a voltage error detectorcomprising four resistors connected in series to form a bridge circuithaving two pairs of opposite junctions, one of said resistors being anonlinear voltage dependent resistor, means connecting one pair of saidopposite junctions to be energized by the direct current voltage of saidfirst rectifier, means connecting the other pair of said oppositejunctions to said voltage control winding for impressing thereon areversible unidirectional voltage varying in direction and magnitudewith variations of said direct current voltage of said first rectifierfrom a predetermined value to vary the impedance of said reactancewindings for maintaining the direct current voltage of said firstrectifier substantially constant at said predetermined value, and timedelay compensation means comprising a transformer having a first windingconnected across said one pair of opposite junctions and a secondwinding connected in series with said voltage control winding.

4. A voltage error detector comprising four resistors connected inseries to form a bridge circuit having two pairs of opposite junctions,two of said resistors being nonlinear voltage dependent resistors, saidnonlinear re sistors being disposed in opposite arms of said bridgecircuit, means connecting one pair of said opposite junctions to avariable voltage source, means connecting the other pair of saidopposite junctions to a voltage responsive element for impressing onsaid element a voltage varying in magnitude with variations of thevoltage of said source from a predetermined value, and time delaycompensation means comprisinga transformer having a first windingconnected across said one pair of opposite junctions and a secondwinding connected in series with said element.

5.-A voltage error detector comprising four resistors connected inseries to form a bridge circuit having two pairs of opposite junctions,two of said resistors being nonlinear voltage dependent resistors, saidnonlinear resistors being disposed in opposite arms of said bridgecircuit, means connecting one pair of said opposite junctions to avariable unidirectional voltage source, means connecting the other pairof said opposite junctions to a voltage responsive element forimpressing on said element a reversible unidirectional voltage varyingin direction and magnitude with variations of the voltage of said sourcefrom a predetermined value, and time delay compensation means comprisinga transformer having a first winding connected across said one pair ofopposite junctions and a second winding connected in series with saidelement.

6. In combination, a first rectifier, a source of alternating currentfor energizing said first rectifier, a first saturable reactor having areactance winding in series with said source and having a bias windingand a signal winding acting in opposition for controlling the directcurrent voltage of said first rectifier, means for impressing asubstantially constant unidirectional voltage on said bias winding, asecond saturable reactor having a second reactance winding, meansincluding a second rectifier connecting said source to said secondreactance winding in series with said signal winding for impressing aunidirectional signal voltage on said signal winding, said secondsaturable reactor having a voltage control winding for controlling themagnitude or" said signal voltage, and a voltage error detectorcomprising four resistors connected in series to form a bridge circuithaving two pairs of opposite junctions, two of said resistors beingnonlinear voltage dependent resistors, said nonlinear resistors beingdisposed in opposite arms of said bridge circuit, means connecting onepair of said opposite junctions to be energized by the direct currentvoltage of said first rectifier, means connecting the other pair of saidopposite junctions to said voltage control winding for impressingthereon a reversible unidirectional voltage varying in direction andmagnitude with variations of said direct current voltage of said firstrectifier from a predetermined value to vary the impedance of saidreactance windings for maintaining the direct current voltage of saidfirst rectifier substantially constant at said predetermined value, andtime delay compensation means comprising a transformer having a firstwinding connected across said one pair of opposite junctions and asecond winding connected in series with said voltage control winding.

N 0 references cited.

